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Map Projections by Randy Schardt, BLM Montana State Office Recorded: April 2, 2009

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Map Projections by Randy Schardt, BLM Montana State Office Recorded: April 2, 2009 Geospatial Learning Pathways Presentation Map Projections by Randy Schardt, BLM Montana State Office recorded: April 2, 2009 This is a Bureau of Land Management Geospatial Learning Pathways Presentation. Map Projections is presented by the Montana State Office. You will learn how to set projection parameters and geographic information system and global positioning system environments. Your presenter is Randy Schardt of BLM’s Montana State Office. Map projections are attempts to portray the surface of the earth or a portion of the earth on a flat surface. And so when a map projection or when we use map projection, will always have distortion of the shape, the area, the distance, and direction. So, no direction can preserve all these properties. Some map projections minimize the distortion to one property at the expense of another while map projections strive to balance the overall distortion. Coordinate systems are a part of map projection. And so what a coordinate system is, it’s a reference system that is used to represent the locations of geographic features, imagery, and observations within a common geographic framework. And it basically, the coordinate system is the definition of the map projection. There are two types of coordinate systems. The first is the geographic coordinate system and the projected coordinate system. Let’s first talk about the geographic coordinate system. The geographic coordinate system is a reference system using latitude and longitude to define the location of points on the surface of a sphere or spheroid. And it can be recorded in several different formats. It can be in decimal degrees, degrees, minutes, seconds, or degrees decimal minutes. In a geographic coordinate system, the earth is not a sphere. The poles are flattened and there’s bulges at the equator. So, basically, the earth becomes a spheroid or an ellipsoid. In a geographic coordinate system, the universal coordinate system is the lat/long and this equates with the longitude values seen with the X axis and the latitude values are with the Y axis. Geographic coordinate system, or the lat/long, is good for locating positions on the surface of the globe, but it is not an efficient means for measuring distance and areas because the latitude and longitude are not uniform units of measure. So if you have data in lat/long, you’ll not be able to calculate acres; you’ll not be able to calculate miles, or anything like that. So you’re going have to reproject that data. In a geographic coordinate system, the datum will define the position of the spheroid relative to the center of the earth. And this is the origin and orientation of latitude and longitude lines are determined by the datum. And there are hundreds of datums customized for different parts of the world. So what common datums do we use in the US? The first one is the North American Datum of 1927 or NAD 27. This uses the Clarke 1866 spheroid. The reference point is located in Mead Branch, Kansas and it’s based on ground survey that was conducted in the 1800’s. Next datum is the North American Datum 1983 or NAD 83. This uses the GRS 80 spheroid and this is based on ground surveys and satellite information. And lastly, we have WGS 1984. This is the most recently developed datum framework or measurements worldwide. It is earth-centered. It has earth-centered perspective and this is the datum that is used by all GPS satellites at this time. WGS 1984 is nearly identical to NAD 83; therefore, NAD 83 is compatible with date collected in GPS using WGS 84. So, both those are comparable, there pretty close to one another. We are trying to get away from MAD 27. We discourage anybody from using NAD 27, that is old technology. So, if you want the most accurate datum, use NAD 83 or WGS 1984. Now, let’s talk about the projected coordinate systems. Projected coordinate system is the systematic transformation of locations on the earth, which are latitude and longitude, to planer coordinates. Projected coordinate system always will use a geographer coordinate system, which references the datum NAD 27 or NAD 83. There are a total of 66 supported map projections. The Montana / Dakotas BLM mandates using the Albers Conic equal area 1983 projected coordinate system. So one of the reasons that we want to use a projected coordinate system over a geographic system. With a projected coordinate system we have the ability to make accurate measurement and to be sure that the spatial analysis options calculate the distance correctly. Projected coordinate system preserves one or more of the following properties: the area, the shape, the distance, and the direction. And with projected coordinate system, we have a better overall appearance of the map. We don’t have the flattening of the data as we do in the geographic coordinate system. So why do we use the Albers projection for the Montana / Dakotas? The Albers projection uses two standard parallels compared with the distortion that a projection with one standard will give you. So although neither the shape nor the linear scale will be truly correct with the Albers. The distortion between the properties is minimized in the region between standard parallel. So, for the Albers projection for Montana, we have two standard parallels. Any data within those two standard parallels - the distortion is going to be minimized. And this projection is best suited for land maps extending in the east to west orientation rather than those lying north and south. So, because we cover three states, basically we’re looking east-west. We use that instead of, maybe, a state like California who has a north-south orientation. One final about Albers that it does not use zones. So we don’t have to worry about multiple zones covering the three states. So here’s, kind of, an example of the different coordinate systems. Up in the upper left, we have the Albers, the Montana BLM custom Albers coordinate system. Over on the right, this is the Montana state plane, 5th zone 25 coordinate system. Well, you notice that for Montana, North and South Dakota, we don’t have just one zone. There are three different state planes zones. So, we would have to decide which zone would look the best. The lower left is the latitude/longitude and notice how it flattens out the data. Then, the lower right, we have the UTM coordinate system and the four zones that would make up for the Montana Dakotas. So, with these examples, the Albers would be the right choice because we don’t have to deal with different zones and the data comes out looking more realistic. So, let’s talk a little about defined coordinate systems. Datasets have a defined coordinate system and can automatically integrate with different coordinate systems by projecting on the fly in ArcMap. What that means is, if we have a projection of Albers and we bring shapefile that has a projection of UTM, ArcMap will automatically reproject on the fly and place the UTM data in the right spot using Albers projection. ArcPad does not reproject on the fly, so it requires all the data to be in the same coordinate system. So, if you have data that has not been defined, you will not be able to use that in ArcPad, because ArcPad will not know the coordinate system and where to place it. If the data has a coordinate system definition but it does not match the BLM coordinate system, you can project that data. To do that, you go to the Project tool, or the Project Raster Tool and the Data Management Tool Box. Or, you can add the data to an ArcMap project; you can set the data view coordinate system to the BLM custom Albers projection and export the data out using the coordinate system of the data frame. So there’s two different ways of doing that and we’ll cover this a little bit later. We talked about defined coordinate systems, let’s talk a little bit about undefined coordinate systems. Undefined coordinate systems, data with undefined coordinate systems will work in ArcGIS. However, you will not be able to take advantage of the functionality in ArcGIS. For example, the shapefile cannot be projected on the fly. It will draw, but it won’t be put in the right spot. If the data frames coordinate system is not the same as the undefined data, the data will not be displayed. Also, if the data hasn’t been defined yet, the metadata will not automatically be updated and it will be incomplete. So, if you have some data and it’s not being displayed or it’s not lining up with the other data, what’s the problem? If your data is not displaying, most likely it’s a projection problem. And if it’s a projection problem, your data has huge errors and the features are likely being displayed in the wrong state or the wrong hemisphere. So it’s thrown out in the middle of nowhere. What would be the cause of this? The data does not have a defined coordinate system. Or the defined coordinate system that has been assigned to it is wrong. Somebody has given it the wrong coordinate system. So, to fix this, you’re going to define the coordinate system by using the defined projection tool, in the data management tool box or in ArcCatalog you can right-click on the data set, choose Properties, select the XY coordinate System Tab and then you will be able to one of three things.
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